Effects of constant flow velocity on endurance swimming and fatigue metabolism in red drum and blackhead seabream

Aquaculture has greater potential for seafood production than wild capture fisheries. To meet the growing demand for seafood, China's marine aquaculture industry has begun building deep-water cages in the open sea. However, under these conditions, fish encounter strong currents and waves, and ensuring their healthy growth is key to the farming process. To address these issues, it is necessary to study the sustained swimming abilities of cultured fish species. Blackhead seabream (Acanthopagrus schlegelii) and red drum (Sciaenops ocellatus) are traditional economic fish species in China; however, their sustained swimming ability under a constant current has been underexplored. Therefore, we examined the endurance swimming ability of three size classes of blackhead seabream and red drum at 20 °C. The fish were then subjected to swimming tests of 0, 30, 60, 90, 120, and 150 min at a constant swimming speed of 0.55 m/s (0.80 m/s), 0.65 m/s (0.90 m/s), and 0.70 m/s (0.98 m/s). The fish were then dissected to obtain muscle, blood, and liver samples; sample metabolite concentrations were measured at six time points, each of which guaranteed five sets of valid data. The results indicated that red drum has a significantly stronger swimming ability, and can be cultured in waters with a short-term flow rate not exceeding 0.75 m/s or 3.5 BL/s. Further, blackhead seabream can be cultured in waters with a flow velocity lower than 0.55 m/s or 2.5 BL/s. The species-related metabolic differences were mainly reflected in the hepatic glycogen and blood glucose concentrations, and those in swimming ability caused by body length were mainly reflected by the hepatic glycogen concentration. The hepatic glycogen concentration had the most significant effect on fish with body lengths >28 cm (P < 0.05). Overall, the experimental results indicated that the liver plays a major role in the physiological level of fish swimming fatigue, providing a direction for further research.